[ClusterLabs Developers] RA as a systemd wrapper -- the right way?

[Ken Gaillot]

On 09/22/2016 10:39 AM, Adam Spiers wrote:
> Ken Gaillot <kgaillot at redhat.com> wrote:
>> On 09/22/2016 08:49 AM, Adam Spiers wrote:
>>> Ken Gaillot <kgaillot at redhat.com> wrote:
>>>> On 09/21/2016 03:25 PM, Adam Spiers wrote:
>>>>> As a result I have been thinking about the idea of changing the
>>>>> start/stop/status actions of these RAs so that they wrap around
>>>>> service(8) (which would be even more portable across distros than
>>>>> systemctl).
>>>>>
>>>>> The primary difference with your approach is that we probably wouldn't
>>>>> need to make the RAs dynamically create any systemd configuration, since
>>>>> that would already be provided by the packages which install the OpenStack
>>>>> services.  But then AFAIK none of the OpenStack services use the
>>>>> multi-instance feature of systemd (foo@{one,two,three,etc}.service).
>>>>
>>>> The main complication I see is that pacemaker expects OCF agents to
>>>> return success only after an action is complete. For example, start
>>>> should not return until the service is fully active. I believe systemctl
>>>> does not behave this way, rather it initiates the action and returns
>>>> immediately.
>>>
>>> But that's trivial to work around: polling via "service foo status"
>>> after "service foo start" converts it back from an asynchronous
>>> operation to a synchronous one.
>>
>> Yes, that's exactly what pacemaker does now: start/stop, then every two
>> seconds, poll the status.
>>
>> However, I'm currently working on a project to change that, so that we
>> use DBus signalling to be notified when the job completes, rather than
>> (or in addition to) polling.
>>
>> The reason is twofold: the two-second wait can be an unnecessary
>> recovery delay in some cases; and (at least from the DBus API, not sure
>> about systemctl status) there's no reliable way to distinguish "service
>> is inactive because the start didn't work properly" from "service is
>> inactive because systemd has some slow-starting dependencies of its own
>> to start first".
> 
> OK, that makes sense - thanks.
> 
>>>> Pacemaker's native systemd integration has a lot of workarounds for
>>>> quirks in systemd behavior (and more every release). I'm not sure
>>>> moving/duplicating that logic to the RA is a good approach.
>>>
>>> What other quirks are there?
>>
>> When pacemaker starts a systemd service, it creates a unit override in
>> /run/systemd/system/<agent>.service.d/50-pacemaker.conf, with these
>> overrides (and removes the file when stopping the resource):
>>
>> * It prefixes the description with "Cluster Controlled" (e.g. "Postfix
>> Mail Transport Agent" -> "Cluster Controlled Postfix Mail Transport
>> Agent"). This gives a clear indicator in systemd messages in the syslog
>> that it's a cluster resource.
>>
>> * "Before=pacemaker.service": This ensures that when someone shuts down
>> the system via systemd, systemd doesn't stop pacemaker before pacemaker
>> can stop the resource.
>>
>> * "Restart=no": This ensures that pacemaker stays in control of
>> responding to service failures.
> 
> Yes, I was aware of that, and you're right that my approach of making
> the RA wrap service(8) or systemctl(8) would need to duplicate this
> functionality - *unless* the creation of the unit override could be
> moved out of Pacemaker's C code into a shell script which both
> Pacemaker and external RAs which want to adopt this wrapping technique
> could call.
> 
>> Additionally:
>>
>> * Pacemaker uses intelligent timeout values (based on cluster
>> configuration) when making systemd calls.
> 
> I guess I'd need more details to fully understand this, but couldn't
> those intelligently chosen timeout values be passed to the RA if
> necessary?  Although that does put a bit of a dampener on my hope of
> using service(8) to remain agnostic to whichever pid-1 system happened
> to be in use on the current machine.  Having said that, maybe everyone
> in the OpenStack (HA) community has already moved to systemd by now
> anyway.

One pacemaker action (start/stop/whatever) may involve multiple
interactions with systemd. At each step, pacemaker knows the remaining
timeout for the whole action, so it can use an appropriate timeout with
each systemd action.

There's no way for the RA to know how much time is remaining.

But I guess it's not important, since pacemaker will timeout the entire
RA action if necessary.

>> * Pacemaker interprets/remaps systemd return status as needed. For
>> example, a stop followed by a status poll that returns "OK" means the
>> service is still running. Fairly obvious, but there are a lot of cases
>> that need to be handled.
> 
> Other than (obviously) start followed by status, what other cases are
> there?

It's just a matter of looking at all the possible return values of each
systemd call, and then mapping that to something the cluster can
interpret. Pacemaker uses the DBus API so the specifics will be
different compared to systemctl. It's just important to get right.

> All of this stuff sounds like generic problems which could be solved
> once for all wrapper RAs via a simple shell library.  I'd happily
> maintain this in openstack-resource-agents, although TBH it would
> probably belong in resource-agents if anywhere.
> 
>> All of these were added gradually over the past few years, so I'd expect
>> the list to grow over the next few years.
> 
> Well, hopefully they could be grown in a way which also supported
> wrapper RAs :-)
> 
> Alternatively, if you think that there's a better solution than this
> wrapper RA idea, I'm all ears.  The two main problems are essentially:
> 
>   1. RAs duplicate a whole bunch of logic / config already provided
>      by vendor packages and systemd service units.
> 
>   2. RAs have a "monitor" action which can do proper application-level
>      monitoring (e.g. HTTP pings), whereas apparently systemd has
>      nothing equivalent.
> 
> So currently we are forced to choose between a) using systemd
> Pacemaker resources, and b) having proper monitoring rather than just
> naive pid-level monitoring, but having to duplicate a whole load of
> stuff which systemd already does nicely.
> 
> If I'm missing something, or you can think of a better alternative
> then please tell me!

I don't see a clear answer.

I suppose a resource-agents interface could minimize the problems.
Something like ocf_start_via_systemd could create an override file,
start a service, and poll until it has a status. Similarly for stop.

The main drawbacks I see are that I'm not sure you can solve the
problems with polling without the dbus interface, and the override file
is tailored to pacemaker (which resource-agents stays independent of).

If you want to give it a try, here are some test cases:

* A service that takes a long time to start, with another resource
ordered after it (make sure the second resource doesn't start until the
first is fully up)

* A service that has a Requires= dependency that takes a long time to
start and is not managed by the cluster

* Use systemctl to shut down the host while the cluster is active, with
resources that take a while to stop